1. Field of the Invention
This invention relates to a method of handling slabs by an overhead traveling crane provided with a slab grip lifter.
2. Description of the Prior Art
Slabs manufactured by a continuous casting line are usually conveyed by a conveyor roller table to a piler delivery table, and piled in a slab yard by an overhead traveling crane having a slab grip lifter. A stack of 10 to 15 slabs is formed in each zone of the slab yard having an address designated in accordance with the nature of the slabs, for example, the subsequent process, the necessity of surface repair, and the manufacturing strand.
Attempts have been made to employ a computer system for the automatic remote control of an overhead traveling crane to accomplish the handling of slabs automatically without relying on any manpower. For the automatic handling of slabs, it is important to ensure that slabs be gripped firmly, and piled without any positional deviation between upper and lower slabs. These attempts have, however, not hitherto met any practical success, partly because of the inability to grip slabs firmly.
For example, if a slab S conveyed by a conveyor roller table to a prescribed position has a centerline L.sub.s disposed at an angle to the centerline L.sub.l of a lifter 2 on an overhead traveling crane moved to the prescribed position to grip the slab S, only a pair of diagonally disposed pawls 1 on the lifter contact the lateral surfaces of the slab S, and the other pair of diagonally disposed pawls 1 do not contact them, as shown in FIG. 1. If the longitudinal centerline L.sub.s of the slab S does not coincide with the centerline L.sub.l of the lifter, though they are parallel to each other, only the two pawls 1 on one side of the lifter contact the adjacent lateral surface of the slab S, and the other two pawls 1 remain spaced apart from the slab S, as shown in FIG. 2.
In either event, however, a powder clutch, which is provided on the lifter to prevent any greater force from acting on the slab brought into contact with any of the pawls, functions to transmit a signal indicating that the slab has been gripped. A powder clutch is a type of electromagnetic clutch which is made by using magnetic material (e.g., iron powder) as a coupling between the input and output rotatable disks in the clutch. A powder clutch is conventional and well known in the art. The clutch functions to transmit driving torque from a motor to a grapple device which may include a lifter, lifter arms and pawls. The lifter is used to grip and lift slabs from different positions in a slab yard.
The lifter is lowered over a slab until the pawls on the lifter arms contact the slab. The lifter is connected to a motor by means such as a rotatable shaft. When the pawls of the lifter contact the slab a reaction force f of the slab against the lifter arms is transmitted to sprocket wheels and a chain through the lifter arms. This force slows the rotation of the rotatable shaft and finally stops its rotation. The moment the shaft stops rotating, a limit switch, for detecting a stop in rotation of the shaft, transmits a signal to turn off the source of power to the motor. Alternatively, the limit switch can break the circuit connected to the source of power to the motor, and thereby stop the motor from turning the shaft when the pawls firmly engage with the slab. As a result, there is every likelihood of the slab S falling from the lifter under the situation shown in FIG. 1, and the lifter cannot grip the slab S under the situation shown in FIG. 2.
Therefore, it has hitherto been essential that a man stay in the slab yard to position the lifter to suit the slab, and give a sign for the hoisting of the lifter after making sure that all of the four pawls of the lifter contact the lateral surfaces of the slab.